So far, fluorescence observation using a microscope, an endoscope, etc. as an apparatus for the fluorescence observation, wherein exciting light is illuminated to a specimen of a living tissue etc., and only the fluorescence generated from the specimen is used, has been carried out.
In order to carry out this fluorescence observation, as for an apparatus for the fluorescence observation as a mechanism for fluorescence observation, an excitation filter which transmits only exciting light of a specific wavelength of illumination light, and an absorption filter which blocks the exciting light and transmits only the fluorescence generated from the specimen when the exciting light is illuminated to the specimen have been used. As another composition for carrying out the fluorescence observation, an absorption filter which blocks exciting light transmits and only the fluorescence generated from a specimen when laser light as exciting light is illuminated to the specimen has been used, wherein the laser light of a specific wavelength obtained from a laser source in which a wavelength selection element, such as etalon and the like, is combined with a dye laser, as illumination light is used.
Conventionally, an endoscope which is used for the fluorescence observation, has been disclosed in Japanese Unexamined Patent Application Toku Kai Hei No. 10-239517, for example. In the endoscope disclosed here, fluorescence observation is carried out by such a way that light having wavelength of 460 nm or less, as exciting light, transmits an excitation filter, and it is irradiated to a living tissue, and the exciting light reflected from the living tissue is cut off by the absorption filter, which transmits the fluorescence having wavelength from about 480 nm to 585 nm generated from the living tissue excited by the exciting light, and an image of this fluorescence is formed on a CCD camera through an image forming optical system.
Generally, the fluorescence generated from a specimen is weak. For this reason, in the fluorescence observation, it becomes important to pick up only the fluorescence efficiently. It is determined by the performance of an excitation filter, an absorption filter, etc. whether only the fluorescence can be picked up efficiently or not.
FIG. 18 is a spectrum characteristic graph showing exemplarily a relation that fluorescence 62 having a longer wavelength range than the wavelength range of exciting light 61 is generated by radiating exciting light 61 to a specimen when the fluorescence observation is carried out by radiating the exciting light 61 having a predetermined spectrum characteristic. The intensity of light of the fluorescence 62 is very weak compared with the intensity of the exciting light 61. As shown in FIG. 18, the wavelength range of the exciting light 61 and the wavelength range of the fluorescence 62 overlap partially. Then, in order to keep the exciting light 61 from overlapping in a wavelength range of peak 62a of the fluorescence 62, it is made to transmit an excitation filter 63, and the light in a wavelength range which does not overlap with the peak 62a of the fluorescence 62 is set as exciting light which is irradiated to the specimen.
The fluorescence 62 is generated by radiating this exciting light 61 to the specimen. In order to pick up and observe the fluorescence only as much as possible, it is desirable that an absorption filter 64 has a portion in which the fluorescence intensity is much at the right and the left sides of the peak 62a of the fluorescence 62 out of generated fluorescence, that is, a spectrum characteristic which transmits light in a wavelength range of the high transmission ratio efficiently. However, since it is desired to avoid that the exciting light irradiated to the specimen enters into the observation side, it is necessary to cut off completely the exciting light by the absorption filter 64. On the other hand, for generating much the fluorescence 62, it is desirable that the excitation filter 63 for determining a wavelength range of the exciting light which is irradiated to the specimen has such spectrum characteristic that the light in a wavelength range (wavelength band) at both sides of the peak 61a of the exciting light 61 is broad as much as possible so as to transmit the exciting light 61 within the wavelength range.
For this, it is desirable that an interval C (wavelength width) is narrow as much as possible and there is no overlapped portion, where the interval C represents an interval between a half-value wavelength A at the long-wavelength side within a wavelength range of the exciting light 61 penetrating the excitation filter 63 (the half-value wavelength A represents a wavelength at the long-wavelength side, wherein a transmittance of the excitation filter 63 becomes a half of the maximum value in the spectrum characteristic graph of FIG. 18. Hereafter, the half-value wavelength A is defined as “a half-value wavelength at the long-wavelength side of an excitation filter”), and a half-value wavelength B at the short-wavelength side in the wavelength range of the fluorescence 62 which transmits the absorption filter 64 (the half-value wavelength B represents a wavelength at the short-wavelength side, wherein transmittance of the absorption filter 64 becomes a half of the maximum value in the spectrum characteristic graph of FIG. 18. Hereafter, a half-value wavelength B is defined as “a half-value wavelength at the short-wavelength side of an absorption filter”). (here, a half-value wavelength means a wavelength when transmittance becomes a half of the maximum transmittance in a spectrum characteristic graph showing a transmittance to each wavelength, that is, it is a wavelength when the transmittance becomes a half)
However, so far, because the performance of the excitation filter and the absorption filter are bad, fluorescence has not been picked up efficiently. For this reason, in order to keep the exciting light from being mixed in the fluorescence, the interval C between the half-value wavelength A at the long-wavelength side of the excitation filter 63 and the half-value wavelength B at the short-wavelength side of the absorption filter 64 is set about 20 nm which is about the wavelength width. Since such light in this wavelength range of about 20 nm has not been used as fluorescence or as exciting light, there is a problem that it has been of no use. This problem is also the same when using laser light as illumination light. That is, since light within a wavelength width between “a wavelength of laser light” and “a half-value wavelength at the short-wavelength side of the absorption filter” has not been used as fluorescence or as exciting light, there was a problem that it has been of no use.
The present invention has been made in consideration of the above-mentioned problem, and it aims at offering an observation apparatus for fluorescence which can pick up weak fluorescence efficiently in the fluorescence observation using an excitation filter and an absorption filter, or in the fluorescence observation using a laser and an absorption filter.